Uranium dioxide or uranium(IV) oxide, also known as urania or uranous oxide,
is an oxide of uranium, and is a black, radioactive, crystalline powder that
naturally occurs in the mineral uraninite. It is used in nuclear fuel
rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used
as MOX fuel. Prior to 1960, it was used as yellow and black color in ceramic
glazes and glass. Production
Uranium dioxide is produced by reducing uranium trioxide with hydrogen.
UO3 + H2 → UO2 + H2O at 700 °C This reaction plays an important part in
the creation of nuclear fuel through nuclear reprocessing and uranium
enrichment. Chemistry
= Structure = The solid is isostructural with
fluorite, where each U is surrounded by eight O nearest neighbors in a cubic
arrangement. In addition, the dioxides of cerium, plutonium and neptunium have
the same structures. No other elemental dioxides have the fluorite structure.
Upon melting, the measured average U-O coordination reduces from 8 in the
crystalline solid, down to 6.7±0.5 in the melt. Models consistent with these
measurements show the melt to consist mainly of UO6 and UO7 polyhedral units,
where roughly 2/3 of the connections between polyhedra are corner sharing and
1/3 are edge sharing. = Oxidation =
Uranium dioxide is oxidized in contact with oxygen to the triuranium octaoxide.
3 UO2 + O2 → U3O8 at 700 °C The electrochemistry of uranium dioxide
has been investigated in detail as the galvanic corrosion of uranium dioxide
controls the rate at which used nuclear fuel dissolves. See the spent nuclear
fuel page for further details. Water increases the oxidation rate of
plutonium and uranium metals. = Carbonization =
Uranium dioxide is carbonized in contact with carbon, forming uranium carbide and
carbon monoxide. UO2 + 4 C → UC2 + 2 CO
This process must be done under an inert gas as uranium carbide is easily
oxidized back into uranium oxide. Uses
= Nuclear fuel = UO2 is used mainly as nuclear fuel,
specifically as UO2 or as a mixture of UO2 and PuO2 called a mixed oxide, in
the form of fuel rods in nuclear reactors.
Note that the thermal conductivity of uranium dioxide is very low when
compared with uranium, uranium nitride, uranium carbide and zirconium cladding
material. This low thermal conductivity can result in localised overheating in
the centres of fuel pellets. The graph below shows the different temperature
gradients in different fuel compounds. For these fuels the thermal power
density is the same and the diameter of all the pellets are the same.
= Color for ceramic glaze = Uranium oxide was used to color glass
and ceramics prior to World War II. Urania-based ceramics become green or
black when fired in a reducing atmosphere and yellow to orange when
fired in an oxidizing atmosphere. Orange-colored Fiestaware is a
well-known example of a product with a urania-colored glaze. Urania has also
been used in formulations of enamel, uranium glass, and porcelain. It is
possible to determine with a Geiger counter if a glaze or glass contains
urania. = Other use =
Depleted UO2 can be used as a material for radiation shielding. For example,
DUCRETE is a "heavy concrete" material where gravel is replaced with uranium
dioxide aggregate; this material is investigated for use for casks for
radioactive waste. Casks can be also made of DUO2-steel cermet, a composite
material made of an aggregate of uranium dioxide serving as radiation shielding,
graphite and/or silicon carbide serving as neutron radiation absorber and
moderator, and steel as the matrix, whose high thermal conductivity allows
easy removal of decay heat. Depleted uranium dioxide can be also
used as a catalyst, e.g. for degradation of volatile organic compounds in gaseous
phase, oxidation of methane to methanol, and removal of sulfur from petroleum. It
has high efficiency and long-term stability when used to destroy VOCs when
compared with some of the commercial catalysts, such as precious metals,
TiO2, and Co3O4 catalysts. Much research is being done in this area, DU being
favoured for the uranium component due to its low radioactivity.
The use of uranium dioxide as a material for rechargeable batteries is being
investigated. The batteries could have high power density and potential of 4.7
V per cell. Another investigated application is in photoelectrochemical
cells for solar-assisted hydrogen production where UO2 is used as a
photoanode. In earlier times, uranium dioxide was also used as heat conductor
for current limitation, which was the first use of its semiconductor
properties. = Semiconductor properties =
The band gap of uranium dioxide is comparable to these of silicon and
gallium arsenide, near the optimum for efficiency vs band gap curve for
absorption of solar radiation, suggesting its possible use for very
efficient solar cells based on Schottky diode structure; it also absorbs at five
different wavelengths, including infrared, further enhancing its
efficiency. Its intrinsic conductivity at room temperature is about the same as
of single crystal silicon. The dielectric constant of uranium
dioxide is about 22, which is almost twice as high as of silicon and GaAs.
This is an advantage over Si and GaAs in construction of integrated circuits, as
it may allow higher density integration with higher breakdown voltages and with
lower susceptibility to the CMOS tunneling breakdown.
The Seebeck coefficient of uranium dioxide at room temperature is about 750
µV/K, a value significantly higher than the 270 µV/K of thallium tin telluride
and thallium germanium telluride and of bismuth-tellurium alloys, other
materials promising for thermoelectric power generation applications and
Peltier elements. The radioactive decay impact of the235U
and238U on its semiconducting properties was not measured as of 2005. Due to the
slow decay rate of these isotopes, it should not meaningfully influence the
properties of uranium dioxide solar cells and thermoelectric devices, but it
may become an important factor for VLSI chips. Use of depleted uranium oxide is
necessary for this reason. The capture of alpha particles emitted during
radioactive decay as helium atoms in the crystal lattice may also cause gradual
long-term changes in its properties. The stoichiometry of the material
dramatically influences its electrical properties. For example, the electrical
conductivity of UO1.994 is orders of magnitude lower at higher temperatures
than the conductivity of UO2.001. Uranium dioxide, like U3O8, is a ceramic
material capable of withstanding high temperatures, making it suitable for
high-temperature applications like thermophotovoltaic devices.
Uranium dioxide is also resistant to radiation damage, making it useful for
rad-hard devices for special military and aerospace applications.
A Schottky diode of U3O8 and a p-n-p transistor of UO2 were successfully
manufactured in a laboratory. Toxicity
Uranium dioxide is known to be absorbed by phagocytosis in the lungs.
See also Uranium oxide
Cleveite Ducrete
References Further reading
Barrett SA, Jacobson AJ, Tofield BC, Fender BEF. "The preparation and
structure of barium uranium oxide BaUO3+x". Acta Crystallographica B 38:
2775–2781. doi:10.1107/S0567740882009935.
External links Semiconducting properties of uranium
oxides Free Dictionary Listing for Uranium
Dioxide The Uranium dioxide International
Bio-Analytical Industries, Inc.